The overall objective of this program is to exploit mutagen-sensitive strains of Drosophila melanogaster to obtain a better understanding the mechanism of eukaryotic DNA metabolism. Within the past year I have developed a system for analyzing by-pass synthesis in larval brain ganglia. This procedure will permit us to assay this function in stocks which must be maintained in heterozygous form. Once the optimum assay conditions have been determined, 17 stocks will be assayed. The kinetics of recovery of DNA synthesis following UV irradiation will be assayed in additional mutants. The synthetic-response will also be monitored immediately after irradiation as a function of dose. In all studies both the level of incorporation and the molecular weight of the product will be monitored. The combined analysis is expected to pinpoint the type of synthetic lesion exhibit by each mutant. Those mutants which exhibit a reduced level of DNA synthesis will be investigated for alterations in chain elongation rates or replicon joining as described in the original proposal. Analysis of the mutant responses to inhibitors of DNA synthesis should also aid identification of the specific lesions. Analysis of unscheduled DNA synthesis will be extended to as many mutants as time permits. Although we have encountered obstacles in this analysis which are unique to Drosophila, they seem now to have been overcome.